Malignant gliomas are the most common primary brain tumour, accounting for the majority of cancers in the adult central nervous system. The incidence of malignant gliomas is increasing world-wide, particularly among the elderly. In general, malignant gliomas are not curable tumours. Traditional pathological approaches grade malignant gliomas into three grades of malignancy. Grade II low grade astrocytoma, with a mean patient survival of 5-15 years. Grade III anaplastic astrocytoma, with a mean patient survival of 3 years. Grade IV glioblastoma multiforme (GBM), the most malignant and common glioma, with a mean survival time of less than 1 year despite aggressive therapy that combines state-of- the-art imaging with surgery, radiotherapy and chemotherapy. Although a transient response to therapy is often observed, tumour recurrence is almost inevitable and usually occurs within tissue that has received intensive cytotoxic therapy, suggesting a sub-population of resistant cells are responsible for tumour regrowth. One of the most prominent topics in the field of cancer biology is that tumour-initiating cells, which phenotypically mimic the cardinal properties of stem cells, are responsible for the origin and maintenance of solid tissue malignancies. The idea that tumor-initiating cells (TICs) may exhibit stem cell characteristics was first confirmed in the 1990s, based on studies of acute myeloid leukaemia and has since been strengthened by findings related to breast, prostate, lung and mesenchymal tumours. The ability of cells derived from human glioma tissue to generate neurospheres in culture suggests the presence of tumour cells with neural stem cell properties (tNSCs) within CNS tumours, as neurosphere generation is one indicator of the presence of neural stem cells. Furthermore, work from ourselves and colleagues have demonstrated the isolation, propagation and serial transplantation of tNSCs that exhibit very similar functional properties as neural stem cells. Importantly, following implantation of tNSCs, the resulting tumours exhibit the classic in vivo features of human glioblastoma multiforme;recapitulating the morphology, genotype and gene expression patterns as primary GBMs, as well as having extensive migratory and infiltrative capacity. This indicates that the in vitro defined brain tumour stem cells faithfully preserve the key in vivo features of human glioblastoma multiforme (hGBM). One of the defining characteristics of somatic stem cells is their infrequent cell division due to cell cycle arrest, which is essential to prevent premature stem cell depletion over the lifetime of the organism. As it is well established that cell cycle arrest protects cells from irradiation and cytotoxic agents, it follows that the slow-cycling TICs will be resistant to conventional therapeutic approaches. In light of these observations, we hypothesise that post-therapy tumour regrowth is initiated from a small population of slow cycling chemo/irradiation-resistant TICs that exhibit the basic stem cell property of self-renewal. TICs then generate a separate and transient population of rapidly dividing progeny cells that are responsible for repopulating the tumour bulk, much like somatic stem cells generate a rapidly dividing pool of precursor cells to replace damaged tissue. Importantly, this model predicts that the elimination of the TICs is required for the long-term recovery of glioma patients. The underlying hypothesis of this proposal is that hGBM tumours contain a population of slow- cycling cells, which can initiate secondary tumour formation and are resistant to conventional treatment. The specific aims will establish the existence of slow cycling hGBM cells in vitro and in vivo, characterize the effects of irradiation and chemotherapy on this sub-population, and investigate the mechanisms underlying this resistance. The outcome of this study will likely define relevant targets, detail its mechanism of resistance and initiate a new avenue for drug development. PUBLIC HEALTH RELEVANCE: Malignant gliomas are the most common primary brain tumor, accounting for the majority of cancers in the adult central nervous system. They are increasing worldwide and carry a poor prognosis with most patients succumbing to their disease regardless of treatment. We are investigating the role of the cells responsible for driving long-term tumor growth and resistance to treatment using methods and techniques commonly used to study normal adult stem cells. The project should unveil the stem cell characteristics of brain tumor cells and provide insight in how to regulate the behavior of the cancer cells.